To assist in securing corrugated or other roofing sheets to an underlying structure, screw threaded fasteners with sealing washers have been developed as have special electric power tools which are usually referred to simply as "screw drivers". Such screw drivers include a clutch which is engaged by the action of pressing the socket at the free end of the drive shaft against the head of the fastener. The socket is within a nose piece which can be adjusted with respect to the body of the tool in the direction of the axis of the power shaft. The nose piece abuts the roofing sheet as the fastener approaches its fully driven-in condition and the drilling pressure exerted is transferred from the fastener to the nose piece. As soon as this happens, the clutch disengages as the requisite force to hold it engaged is no longer present. In practice, the fastener advances slightly after the nose piece abuts the roofing sheet. If the nose piece has been correctly set, and the roofing sheet is standard insofar as the manner in which it deforms when the nose piece engages it is concerned, then the fasteners will be properly secured as the clutch will disengage at the right moment. However, setting the clutch accurately is extremely difficult and roofing sheets vary very substantially in characteristics. Furthermore, the nose piece is small and thus drilling pressure acts on a small area of the roofing sheet. Invariably, therefore, some of the fasteners on the roof tend to be overdrilled. This is because either the clutch did not disengage at the right moment, but remained engaged for too long, or the roofing sheet gave more than expected. The result is a depressed area around the fastener which can fill with water and cause costly leaks. To compensate for this an experienced worker sometimes judges what the characteristics of the sheet are and stops his tool before the clutch disengages. This, if not correctly and carefully done, results in under-drilling which means that the fastener is loose and not properly sealed.
The problems set forth above are particularly severe when the sheet is fastened through its ridges and not through its valleys. This is done in hot climates to permit greater freedom of distortion during sudden temperature changes. Fasteners driven through the sheet valleys tend to shear-off if the sheets are subjected to great temperature changes.
A further difficulty is that the fasteners are usually quite long, for example, 50 or 75 mm. With a conventional screw driver the fasteners are only held at their head end and, immediately pressure is applied, they tend to tilt over about their point of contact with the roofing sheet. Thus, it is difficult to guide the fastener and considerable skill is required on the part of the operator.
The present invention provides an attachment which overcomes, or at least minimises, both these difficulties.